Fuel cells are electrochemical energy conversion devices that generate electricity and heat by converting the chemical energy of fuels. A single fuel cell normally consists of an electrolyte sandwiched between two electrodes, a porous anode and a cathode. While a variety of different fuel cell types have been developed, all operate on essentially the same principle. For a PEM fuel cell, hydrogen, or a hydrogen-rich fuel is fed to the anode where a catalyst separates the hydrogen's negatively charged electrons from positively charged ions (protons). The electrons from the anode side of the cell cannot pass through the membrane to the positively charged cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This movement of electrons is an electrical current which is advantageously used to drive a load, such as an electric motor or other electrical system. Once delivered to the cathode via the electrical circuit, the electrons combine with the protons that have crossed the membrane and the oxygen from the air, resulting in water or hydroxide. For proton exchange membrane (PEM) and phosphoric acid fuel cells, protons move through the electrolyte to the cathode to combine with oxygen and electrons, producing water and heat. In other types of fuel cells such as solid oxide fuel cells (SOFC's), negative ions travel through the electrolyte to the anode where they combine with the hydrogen or other oxidizable “fuel”
In the case of hydrogen fuel cells, hydrogen fuel may be fed to the anode in what is sometimes referred to as the anode loop. The quantity of hydrogen fed to the anode is a function of variety of factors, including the relative purity of hydrogen fuel, low demand and other variable parameters that are unique to each fuel cell application.
In order to operate efficiently, the fuel cell must be supplied with more hydrogen fuel then it can actually convert. As a result, extra, unused hydrogen gas is discharged from the fuel cell. In order to increase operating efficiencies, it has been proposed that the unused hydrogen gas be circulated and combined with fresh gas from the hydrogen source before being redelivered to the anode of the fuel cell. Known hydrogen recirculating systems rely on comparatively complicated mechanical components, or electrical control systems that expose sensitive electronic components to potentially harsh environments found in fuel cells. It would therefore be desirable to provide a system for recirculating unused hydrogen fuel that is both simple in construction and well suited to operate within the adverse environment of the fuel cell. The present invention is directed toward satisfying this need.